The overall hardware performance of a multiprocessor system is generally dependent on three factors: the productive processing power of each processor, the communications bandwidth of the interconnection amongst processors, and the communications latency amongst processors. Processing power continues to grow, as does communications bandwidth. Since the third factor, latency, is a function of the speed at which a signal can travel, communications latency will eventually be limited by the speed of light in vacuum (unless the current understanding of physics changes). Electrical signals are already being propagated, along high grade copper cables, at up to 80% the speed of light in vacuum. As such, there is not much room left for improvement. Consequently, as processor power and bandwidths continue to improve, the lack of corresponding improvement in communications latency will increasingly become the main limiting factor on the overall performance of a multiprocessor system.
One of the ways to reduce communications latency is to place processors increasingly closer together, so that the distances signals travel can be shortened. Correspondingly and positively, this also increases the amount of computing power in a unit volume (i.e., the “computational density”). As computational density increases, however, it can be difficult to dissipate the heat generated by processors placed in close proximity.
Multiprocessor computer designers are therefore faced with the problem of reducing communications latency and increasing computational density while at the same time ensuring adequate heat dissipation.